a) Field of the Invention
The present invention relates to a wide-angle lens system which has a field angle on the order of 50.degree. to 90.degree., an F number on the order of 4.0 or high brightness and a long back focal length, and is optimum for use in the electronic cameras and video cameras which use image pickup tubes and solid-state image pickup devices.
b) Description of the Prior Art
For the electronic cameras and video cameras which use image pickup tubes and image pickup devices, it is necessary to dispose optical members such as low pass filters and infrared cut filters between lens systems and surfaces of the image pickup devices. Therefore, lens systems to be used in these cameras must have back focal lengths which are long as compared with focal lengths thereof.
In case of an image pickup system which uses a color separating optical system for picking up three colors of R, B and G with a plurality of image pickup devices to improve qualities of colored images, it is necessary to interpose optical elements such as mirrors and prisms for splitting an optical path in addition to the optical elements such as the low pass filter, whereby a lens system must have a longer back focal length.
Lens systems disclosed by Japanese Patents Kokai Publication No. Sho 63-149618, Kokai Publication No. Hei 1-61714 and Kokai Publication No. Hei 4-118612 are known as conventional examples of wide-angle lens systems which have field angles not smaller than 50.degree. and back focal lengths long enough for interposing optical elements such as mirrors and prisms for splitting optical paths.
In the recent years where progress has been made in manufacturing technologies, image pickup devices which are used in electronic cameras and video cameras have been shifted from image pickup tubes mainly to solid-state image pickup devices. Solid-state image pickup devices which have a large number of pixels are available.
Accordingly, cameras which use solid-state image pickup devices are now usable for printing purposes though these cameras were not used conventionally for a reason that images obtained with these cameras were lower in quality than those of images obtained with silver salt cameras. However, it is difficult to manufacture a compact image pickup device even with the recent manufacturing technologies since it requires pixels in a number equal or larger than that of pixels specified by standards for televisions such as highvision which forms highly minute images.
Therefore, electronic cameras are being developed by arranging larger numbers of pixels on enlarged image pickup devices without changing a size of each pixel.
However, when enlarged image pickup devices can be manufactured from a single wafer which has a definite area and require a higher manufacturing prime cost. Accordingly, attempts have been made to accomplish both a compact configuration and a reduction of manufacturing costs at the same time by reducing the size of pixels and developing an image pickup device on which a large number of pixels are arranged.
However, the reduction in a size of pixels to be arranged on an image pickup device results in enhancement of the so-called Nyquist frequency, thereby requiring a photographic lens system which has extremely high optical performance.
On the other hand, the electronic cameras and video cameras which use electronic image pickup devices require an optical system having a long back focal length, thereby obliging to use a retrofocus type optical system which has a negative-positive power distribution in order from the object side, or a negative-positive power distribution asymmetrical with regard to a stop. As a result, the optical system can hardly correct offaxial aberrations such as distortion and astigmatism, a paraxial light bundle is made a diverging light bundle by a front lens group and spherical aberration is produced by a rear lens group, thereby making it difficult to obtain a bright lens system.
Above all, a lens system which has a wider photographing field angle has a back focal length having a higher ratio relative to a focal length thereof, thereby making it necessary to strengthen powers of a negative lens group and a positive lens group or widening an airspace between the negative lens group and the positive lens group.
The former method further aggravates aberrations, thereby making it difficult to maintain high optical performance of the lens system. Further, the latter method enlarges the lens system. In order to obtain a lens system which has a relatively compact size, a long back focal length and high optical performance, it is therefore necessary to select an adequate negative-positive power distribution as well as an adequate value of distance between principal points.
Since problems of color reproducibility, color moire, etc. are more serious as an image pickup system is configured to reproduce images of higher qualities, a multi-plate camera represented by the so-called three-plate camera which picks up the three primary colors RGB with three image pickup devices is more desirable than the so-called single-plate camera which uses a single image pickup device on which color filters are arranged in a mosaic or stripes.
For this reason, it is necessary to interpose a color separating optical system between a photographic lens system and image pickup devices, thereby requiring a much longer back focal length and making it more difficult to design a photographic lens system.
Lens systems disclosed by Japanese Patents Kokai Publication No. Sho 63-149618 and Kokai Publication No. Hei 1-61714 are known as conventional examples of photographic lens systems which are used in such a image pickup system. These lens systems do not sufficiently correct spherical aberration and astigmatism. Though a lens system which is disclosed by Japanese Patent Kokai Publication No. Hei 4-118612 which is known as another conventional example has remarkably high optical performance, this lens system is composed of a remarkably large number of lens elements, whereby the lens system is large as compared with a focal length thereof and is contrary to the object described above of an image pickup system which is configured compact by reducing a size of image pickup devices.